[Python] yield 的用法

這幾篇寫的都蠻清楚的
https://www.jianshu.com/p/d09778f4e055
http://pyzh.readthedocs.io/en/latest/the-python-yield-keyword-explained.html
https://www.jianshu.com/p/d676543ccdbb
https://jeffknupp.com/blog/2013/04/07/improve-your-python-yield-and-generators-explained/